A semiconductor light emitting element is formed of several layers such as a light emitting layer, an n-type semiconductor layer, a p-type semiconductor layer, an electrode layer, and a supporting substrate. Therefore, the light emitted in the light emitting layer inside the semiconductor element is extracted to the outside after having passed through these several layers. However, when the light passes through a boundary between media having different refractive indexes, i.e., an interface between layers or a surface, a certain rate of reflection occurs inevitably. In addition, when the light passes through or is reflected from a medium layer having an absorption coefficient for a wavelength (emission wavelength) of the aforementioned light, a certain rate of light absorption occurs. Therefore, it is generally difficult to efficiently extract the light emitted in the light emitting layer to the outside of the semiconductor light emitting element.
Particularly when the light travels from a medium having a high refractive index to a medium having a low refractive index, total reflection of the light occurs and the light having an angle equal to or larger than a critical angle cannot be extracted to the outside. On the surface of the semiconductor light emitting element, i.e., an interface between the air (or a sealing material) and the semiconductor element, a difference in refractive index between the media is large, and thus, the critical angle at which total reflection occurs becomes small, which results in an increase in ratio of the light totally reflected from the interface.
For example, a refractive index n of a sapphire substrate is 1.8 and a critical angle to the air is 33.7 degrees. Namely, when a sapphire substrate is used as the substrate forming the semiconductor light emitting element and when the light is extracted through the sapphire substrate to the air side, the light having an incidence angle larger than 33.7 degrees is totally reflected and cannot be extracted to the outside. In the case of an aluminum nitride (AlN) substrate having a larger refractive index (refractive index n=2.29), a critical angle is 25.9 degrees and only a smaller amount of the light can be extracted to the outside.
By using theoretical calculation of the light radiation propagation property with a finite-difference time-domain method (FDTD method), the light extraction efficiency in a semiconductor light emitting element having an AlGaN layer stacked on an AlN substrate was calculated, for example. As a result, considering absorption or the like by a p-type GaN layer located opposite to the AlN substrate as seen from a light emitting portion inside the AlGaN layer, the extraction efficiency of the light that can be extracted from the surface (light extraction surface) side of the AlN substrate, of the light with a wavelength of 265 nm radiated from the light emitting portion, is extremely low, i.e., about 4%.
In order to deal with the aforementioned problem, there has been proposed a semiconductor light emitting element having a nanometer-scale recessed and projecting structure on a substrate surface (light extraction surface). For example, PTD 1 discloses that a light extraction surface is provided with a recessed and projecting structure having an average period that is not more than twice as great as an average optical wavelength of the light emitted from a light emitting layer. PTD 1 proposes a method for reducing a ratio of the light totally reflected from the light extraction surface (i.e., suppressing reflection of the light from the element surface), by forming the aforementioned recessed and projecting configuration. However, it is not easy to form the nanometer-scale recessed and projecting structure on the surface of the semiconductor light emitting element. In addition, the light extraction efficiency greatly varies depending on the shape of the recessed and projecting structure and the emission wavelength, and thus, the effect is not sufficiently obtained.
As the emission wavelength becomes shorter, the required period of the recessed and projecting structure (e.g., in the case of a projecting structure, a distance between a vertex portion of a projecting structure and a vertex portion of an adjacent projecting structure) becomes shorter, and thus, fabrication of the recessed and projecting structure becomes difficult. Particularly in a semiconductor light emitting element that emits the light in an ultraviolet or deep ultraviolet wavelength range, it is difficult to fabricate the recessed and projecting structure of such size by optical lithography. As a result, problems such as an increase in fabrication cost and a reduction in yield and productivity arise, and thus, fabrication of such recessed and projecting structure is not practical.
PTD 1 (Japanese Patent Laying-Open No. 2005-354020) discloses a method for heating and flocculating a deposited metal to form a nanometer-sized minute metal mask, forming the nanometer-sized minute metal mask on the light extraction surface, and etching a surface of the light extraction surface, in order to form the nanometer-scale periodic recessed and projecting structure. However, in the case of the aforementioned periodic mask using the flocculation effect, arrangement of the recessed and projecting structure is random and the non-uniformity of a shape thereof is high. Therefore, variations in power of the light output from the semiconductor light emitting element to the outside are large and it is difficult to provide a semiconductor light emitting element that emits the stable and uniform light.
NPD 1 (ISDRS 2011, Dec. 7-9, 2011, College Park, Md., USA, WP2-04) discloses a method for roughening a substrate surface by wet etching in order to form a nanometer-scale recessed and projecting structure. However, the recessed and projecting structure formed by a method using wet etching also has a random structure having a non-uniform shape. Therefore, the light extraction efficiency greatly varies and the effect of enhancing the light extraction efficiency is also insufficient.
According to NPD 2 (Appl. Phys. Express 3 (2010) 061004), in a semiconductor light emitting element that emits the deep ultraviolet light, a surface periodic recessed and projecting structure is provided by lithography and dry etching. However, a period of the recessed and projecting structure is 500 nm, which is approximately twice as great as an emission wavelength, and the effect of enhancing the light extraction efficiency is not sufficiently obtained. In addition, variations in light output are extremely large.